1. Field of Invention
The present invention relates to piezoelectric transformers. More particularly, the present invention relates to a multi-layer piezoelectric transformer used in a circuit to generate a voltage sufficient to create a discharge across the spark gap, for example in an internal combustion engine of high intensity discharge lamp.
2. Description of the Prior Art
Wound-type electromagnetic transformers have been used for raising input voltages (step-up transformation) in internal power circuits of devices that require high voltage such as televisions, charging devices of copier machines or fluorescent lamp ballasts. Such electromagnetic transformers take the form of a conductor wound onto a core made of a magnetic substance. Because a large number of turns of the conductor are required to realize high transformation ratios, electromagnetic transformers that are effective, yet at the same time compact and slim in shape are extremely difficult to produce.
In particular, wound transformers have been used in the ignition systems of automobiles. The purpose of the ignition system is twofold: first to create a voltage high enough (20,000+) to arc across the gap of a spark plug, thus creating a spark strong enough to ignite the air/fuel mixture for combustion; second to control the timing so that the spark occurs at the right time and at the right cylinder.
The automotive ignition system includes designs from a mechanical system (distributor) to a solid state electronic system. The ignition circuit consists of two sub-circuits: the primary, which carries low voltage; and the secondary, which carries high voltage. The low voltage primary circuit will induce a high voltage in the secondary circuit through an ignition coil. The high voltage is then directed through the secondary circuit to the right spark plug at the right time.
The coil is a compact, electrical transformer that boosts the battery's 12 volts to as high as 20,000 volts. The incoming 12 volts of electricity pass through a primary winding which typically contains from 100 to 250 turns of heavy copper wire which raises the power to about 250 volts. The turns of this wire must be insulated from each other or they would short out and not create the primary magnetic field that is required. The primary circuit, controlled by the ignition key, releases 12 volts of electricity from the battery or alternator through the coil to a set of breaker points in the lower part of the distributor, or to the relay in electronic ignition applications. When the points or relay are closed, current flows through the chassis back to the battery, completing the circuit. When the points or relay are open, each interruption causing a breakdown in the coil's electromagnetic field. Each time the field collapses, a surge of electricity passes to a secondary winding. The coil secondary winding circuit typically contains from 15,000 to 30,000 turns of fine copper wire, which also must be insulated from each other. When the field collapses, the current is boosted to the high voltage needed for ignition and is then relayed to the rotor. To further increase the coil's magnetic field both windings are installed around a soft iron core. To withstand the heat of the current flow, the coil is filled with oil for cooling. As current flows through the coil a strong magnetic field is built up. When the current is shut off, the collapse of this magnetic field induces a high voltage which is released through the large center terminal of the ignition coil through the distributor to the spark plugs.
The secondary circuit consists of the secondary windings of the ignition coil, which produces the high voltage needed to arc across the spark plug gap. This voltage is sent out the center coil tower, through a high-tension (high voltage) wire to the distributor cap where a rotor distributes the spark through the distributor cap, to the right spark plug at the right time.
The distributor is separated into three sections: the upper, middle, and lower. In the middle section, the corners of the spinning breaker cam strike the breaker arm and separate the points. High-voltage surges generated by the action of the coil travel to the rotor that is rotating inside a circle of high-tension terminals in the distributor cap. At each terminal, current is transferred to wires that lead to the spark plugs. Two other devices--the vacuum advance and the centrifugal advance--precisely coordinate the functions of the points and the rotor assembly as the requirements of the engine vary.
Many older vehicles use a breaker point system to control the timing of the ignition system. Breaker points have not been used since the mid 70's, having been replaced by electronic ignition systems, but many older cars still have them. The points are made up of a fixed contact point and a movable contact point. The movable point is spring loaded and rides on a 4,6, or 8 lobe cam (depending on the number of cylinders). The points are located inside a distributor. As the engine rotates, the camshaft turns the distributor, which then opens and closes the breaker points as many as 15,000 to 25,000 times a minute. When the points are closed, current is allowed to flow through the ignition coil, thereby building a magnetic field around the windings. When the points are opened, they interrupt that current flow, thereby collapsing the magnetic field and releasing a high voltage surge. This high voltage enters the top of the distributor, where an ignition rotor distributes that voltage through a cap to the right spark plug at the right time. The distributor also contains a condenser that prevents arcing by absorbing excess current when the points open.
The difficulty with the breaker point system is that the part that rubs against the cam wears. This wear causes a constant need for adjustment and eventual replacement. In the mid 70's this problem was corrected through the use of solid state electronics and transistors as switching devices.
Electronic ignition systems also exist. Instead of having a wear-prone and super-sensitive mechanical switch control the flow of current to the primary winding of the coil, transistors were used. These semiconductor devices have the ability to switch a relatively large amount of amperage when they receive a very small control signal current, they are for all practical purposes capable of doing this at any speed, and they don't wear or erode.
The various breakerless electronic ignition system designs differ in the way the transistor control signal current is produced. The earliest was the magnetic pickup type, in which a reluctor or armature having as many teeth as there are cylinders in the engine rotates past a fixed magnet and pickup coil. As each tooth passes the pickup, it generates an electrical impulse (this is similar to an alternator without diodes). The transistor in the control module, also known as the igniter, switches the ignition coil's primary circuit on and off according to this impulse. It is still widely used on many makes today.
The Hall Effect type is also popular. The Hall principle states that if a constant current flows through a thin conducting material, and that material is exposed to a magnetic field, voltage will be generated, and the amount of voltage will depend on the strength of the field. In an ignition system, this phenomenon is put to work by the action of a shutter wheel in the distributor that interrupts the field created within the Hall generator by a permanent magnet (this is mounted across an air gap from a magnetically conducting element with a thin semiconductor layer). The wheel has as many shutters as there are cylinders, and they can be compared to the flat spots on a distributor cam. The ignition control unit is designed to supply current to the coil's primary winding while the shutter wheel is blocking the field. When the shutter moves out of the way and Hall voltage is produced, the control unit cuts off primary circuit current, the coil's field collapses, and induction produces high voltage in the secondary winding. There are other types too, such as optical triggers, but the magnetic pickup and Hall Effect varieties are the most common.
The 20,000 or more volts produced in the coil gets from the distributor cap to the spark plug through spark plug wires. Spark plug wires are made of various layers of materials. The fiber core, inside the spark plug wire carries the high voltage. Resistor spark plugs or special resistor type ignition cable may be used. To work effectively in modern ignition systems, it is important that the resistor ignition cable is capable of producing a specifically designed resistance. The cable must also have enough insulation so that it can withstand heat, cold, moisture, oil, grease, and chafing. High tension electricity passing through a cable builds up a surrounding electrical field. The electrical field frees oxygen in the surrounding air to form ozone, which will attach to the rubber insulation if it is not properly protected. Ozone causes the rubber to deteriorate and lose its insulating qualities. Electrical losses will seriously weaken the spark at the plug gap.
A problem with prior ignition systems is that magnetic coils require a large number of turns of wire to produce a high voltage, and therefore have a large size.
Another problem with prior ignition systems is because of the large size of magnetic coils, each spark plug could not have its own high voltage source.
Another problem with prior ignition systems is because of the large size of magnetic coils, a high voltage distribution system using high tension wires is necessary.
Another problem with prior ignition systems is because of the necessity for a high voltage distribution system, electrical and magnetic interference is generated.
Another problem with prior ignition systems is because of the necessity for a high voltage distribution system wires of specific resistance are necessary to minimize interference.
To remedy this problem, piezoelectric transformers utilizing the piezoelectric effect have been provided in the prior art. In contrast to the general electromagnetic transformer, the piezoelectric ceramic transformer has a number of advantages. The size of a piezoelectric transformer can be made smaller than electromagnetic transformers of comparable transformation ratio. Piezoelectric transformers can be made nonflammable, and they produce no electromagnetically induced noise.
The ceramic body employed in prior piezoelectric transformers takes various forms and configurations, including rings, flat slabs and the like. A typical example of a prior piezoelectric transformer is illustrated in FIG. 1. This type of piezoelectric transformer is commonly referred to as a "Rosen-type" piezoelectric transformer. The basic Rosen-type piezoelectric transformer was disclosed in U.S. Pat. No. 2,830,274 to Rosen, and numerous variations of this basic apparatus are well known in the prior art. The typical Rosen-type piezoelectric transformer comprises a flat ceramic slab 110 which is appreciably longer than it is wide and substantially wider than thick. As shown in FIG. 1, a piezoelectric body 110 is employed having some portions polarized differently from others. In the case of the prior transformer illustrated in FIG. 1, the piezoelectric body 110 is in the form of a flat slab which is considerably wider than it is thick, and having greater length than width. A substantial portion of the slab 110 the portion 112 to the right of the center of the slab, is polarized longitudinally, whereas the remainder of the slab is polarized transversely to the plane of the face of the slab. In this case the remainder of the slab is actually divided into two portions, one portion 114 being polarized transversely in one direction, and the remainder of the left half of the slab, the portion 116 also being polarized transversely but in the direction opposite to the direction of polarization in the portion 114.
In order that electrical voltages may be related to mechanical stress in the slab 110, electrodes are provided. If desired, there may be a common electrode 118, shown as grounded. For the primary connection and for relating voltage at opposite faces of the transversely polarized portion 114 of the slab 110, there is an electrode 120 opposite the common electrode 118. For relating voltages to stress generated in the longitudinal direction of the slab 110, there is a secondary or high-voltage electrode 122 cooperating with the common electrode 118. The electrode 122 is shown as connected to a terminal 124 of an output load 126 grounded at its opposite end.
In the arrangement illustrated in FIG. 1, a voltage applied between the electrodes 118 and 120 is stepped up to a high voltage between the electrodes 118 and 122 for supplying the load 126 at a much higher voltage than that applied between the electrodes 118 and 120.
An inherent problem of such prior piezoelectric transformers that they have relatively low power transmission capacity. This disadvantage of prior piezoelectric transformers relates to the fact that little or no mechanical advantage is realized between the driver portion of the device and the driven portion of the device, since each is intrinsically a portion of the same electroactive member. This inherently restricts the mechanical energy transmission capability of the device, which, in turn, inherently restricts the electrical power handling capacity of such devices. Additionally, because the piezoelectric voltage transmission function of Rosen-type piezoelectric transformers is accomplished by proportionate changes in the x-y and y-z surface areas (or, in certain embodiments, changes in the x-y and x'-y' surface areas) of the piezoelectric member, which changes are of relatively low magnitude, the power handling capacity of prior circuits using such piezoelectric transformers is inherently low.
Because the typical prior piezoelectric transformer accomplishes the piezoelectric voltage transmission function by proportionate changes in the x-y and y-z surface areas (or, in certain embodiments, changes in the x-y and x'-y' surface areas) of the piezoelectric member, it is generally necessary to alternatingly apply positive and negative voltages across opposing faces of the "driver" portion of the member in order to "push" and "pull", respectively, the member into the desired shape. Prior electrical circuits which incorporate such prior piezoelectric transformers are relatively inefficient because the energy required during the first half-cycle of operation to "push" the piezoelectric member into a first shape is largely lost (i.e. by generating heat) during the "pull" half-cycle of operation. This heat generation corresponds to a lowering of efficiency of the circuit, an increased fire hazard, and/or a reduction in component and circuit reliability. Furthermore, in order to reduce the temperature of such heat generating circuits, the circuit components (typically including switching transistors and other components, as well as the transformer itself) are oversized, which reduces the number of applications in which the circuit can be utilized, and which also increases the cost/price of the circuit.
Another problem with prior piezoelectric transformers is, because the power transmission capacity of such prior piezoelectric transformers is low, it is necessary to combine several such transformers together into a multi-layer "stack" in order to achieve a greater power transmission capacity than would be achievable using one such prior transformer alone. This, of course, increases both the size and the manufacturing cost of the transformer; and the resulting power handling capacity of the "stack" is still limited to the arithmetic sum of the power handling capacity of the individual elements.
Accordingly, it would be desirable to provide a piezoelectric transformer design that has a higher power transmission capacity than similarly sized prior piezoelectric transformers.
It would also be desirable to provide a piezoelectric transformer that can generate a voltage sufficient to generate a spark across a spark plug gap in an ignition system.
It would also be desirable to provide a piezoelectric transformer that is smaller than prior piezoelectric transformers that have similar power transmission capacities.
It would also be desirable to provide a piezoelectric transformer in which the "driver" portion of the device and the "driven" portion of the device are not the same electro-active element.
It would also be desirable to provide a piezoelectric transformer that develops a substantial mechanical advantage between the driver portion of the device and the driven portion of the device.
It would also be desirable to provide a piezoelectric transformer that, at its natural frequency, oscillates with greater momentum than is achievable in comparably sized prior piezoelectric transformers.
A problem with prior piezoelectric transformers is that they are difficult to manufacture because individual ceramic elements must be polarized at least twice each, and the directions of the polarization must be along different axes.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because it is necessary to apply electrodes not only to the major faces of the ceramic element, but also to at least one of the minor faces of the ceramic element.
Another problem with prior piezoelectric transformers is that they are difficult to manufacture because, in order to electrically connect the transformer to an electric circuit, it is necessary to attach (i.e. by soldering or otherwise) electrical conductors (e.g. wires) to electrodes on the major faces of the ceramic element as well as on at least one minor face of the ceramic element.
Another problem with prior piezoelectric transformers is that the voltage output of the device is limited by the ability of the ceramic element to undergo deformation without cracking or structurally failing. It is therefore desirable to provide a piezoelectric transformer which is adapted to deform under high voltage conditions without damaging the ceramic element of the device.
It is another problem of prior piezoelectric transformers that they tend to break down (i.e. short) under relatively low voltages.
It is another problem with prior transformers that they have low power utilization efficiencies, such as magnetic transformers which have an efficiency loss of up to 40-50%.
Another problem with prior transformers is that the magnetic core and coiled wire can generate magnetic fields that interfere with surrounding circuitry.
Another problem with prior transformers is that they are difficult to miniaturize to the extent that they could be attached to a spark plug.